I´m no expert, but some of the main problems people have when writing are small things like using "i" instead of "I", "loose" instead of "lose", "specially" and not "especially".
Many people don´t use enough paragraphs either. It´s easier for an examiner to read something that has proper pauses between each idea. Anyway, I got 8 in Writing the first time I took the test and my only preparation was watching a couple of YouTube videos. I used pencil by the way and my hand writing is horrible.
I had to take the IELTS again though, as I did not reach 8 in Listening which is the one that matters the most.